1. Field of the Invention
The present invention relates to a polymer modified asphalt composition. More specifically, but not by way of limitation, the present invention relates to the use of tubular reactor produced ethylene/alkyl acrylate copolymer preferably in combination with an epoxy functionalized ethylene/alkyl acrylate copolymer as polymer modifier for asphalt.
2. Description of the Related Art
It is generally known in the art and an accepted commercial practice to modify asphalt sold for paving applications by the addition of various polymers. Typically polymer additives are employed to improve rut resistance, fatigue resistance, and cracking resistance and sometimes to improve stripping resistance from the aggregate. These improvements tend to result from or correlate to increases in asphalt elasticity and stiffness upon addition of the polymer. Asphalts for paving are performance graded by a set of SHRP (Strategic Highway Research Program) specifications. For example, a PG58-34 asphalt should provide good rut resistance at 58° C. and good cracking resistance at −34° C. The addition of the polymer to the asphalt is known to significantly increase the first number (i.e., provides higher temperature rut resistance) and significantly improve fatigue resistance. The improvements in rut and fatigue resistance result from increases in stiffness and elasticity. In commercial applications, these increases are to be accomplished with relatively small amounts of polymer (i.e., 1 to 5 weight percent polymer). However, such small amounts of polymer do not sufficiently improve the low temperature properties to meet acceptable performance grades, although the properties are somewhat improved. Thus, good low temperature properties for asphalts are mainly obtained by addition of oils such as gas oil. Currently, the commercial polymer modified asphalt (PMA) market is dominated by the use of styrene/butadiene/styrene (SBS) type block copolymer additives. Typically the asphalt industry considers polymers for asphalt modification to be either categorically an elastomer or a plastomer. The term plastomer carries a negative connotation in that a plastomer lacks elastomeric properties. However, plastomers are sometimes used to modify asphalt because they can increase stiffness and viscosity which improves rut resistance but they are generally considered inferior to elastomers due to lack of significant improvements in fatigue resistance, creep resistance, cold crack resistance and the like. SBS polymer is considered an elastomer. There are mixed feelings in the industry as to how to categorize some polymeric asphalt additives.
For example, German patent 1,644,771 discloses and claims bitumen compositions made up of from 5 to 95 wt % aromatic petroleum asphalt and from 95 to 5 wt % of an ethylene/acrylate ester copolymer. The copolymer fraction is either an ethylene/alkyl acrylate or ethylene/alkyl methacrylate copolymer derived from copolymerization of ethylene and from 1 to 40 wt % of alkyl acrylate or alkyl methacrylate ester, wherein the alkyl groups contains from 1 to 8 carbon atoms. At these high concentrations, the ethylene/alkyl acrylate and ethylene/alkyl methacrylate would not be considered as acceptable polymeric additives for producing commercial grade polymeric modified asphalt (PMA) to be used in road paving applications.
In U. S. Pat. Nos. 5,306,700 and 5,556,900, PMA compositions useful in road paving and roofing applications are disclosed. These compositions include a reactive polymeric asphalt additive that chemically reacts and links to the asphalt as a result of the presence of an epoxy functional group. The reactive polymer additive is an ethylene copolymer of the general formula E/X/Y/Z where E represents the ethylene derived unit and constitutes from 20 to 99.5 wt % of the copolymer. The X can be present up to 50 wt % and is derived from, for example, alkyl acrylates, alkyl methacrylates, vinyl esters, or alkyl vinyl ethers. The Y is present in from 0.5 to 15 wt % and is to be derived from, for example, glycidyl acrylate, glycidyl methacrylate or glycidyl vinyl ether. The Z is optionally present up to 15 wt % and is derived from other monomers such as carbon monoxide, sulfur dioxide, acrylonitrile and the like. Of particular note is the reactive terpolymer ethylene/n-butyl acrylate/glycidyl methacrylate (EnBAGMA), which is known (after chemical linking to the asphalt) to significantly improve both elasticity and stiffness of the resulting PMA, much like elastomeric asphalt additive.
In U.S. Pat. Nos. 6,117,926 and 6,399,680, improved PMA compositions are taught wherein an asphalt and a stiffness-enhancing copolymer having available epoxy groups are reacted in the presence of an effective amount of an acid (e.g., H3PO4 and H2SO4) to promote chemical bonding between the asphalt and the available epoxy groups of the copolymer. The use of the acid is shown to minimize the amount of epoxy functionalized polymer additive (generally the most expensive component) required to achieve greater stiffness values over similar reaction times relative to no acid being present. The references also acknowledge that low temperature SHRP performance grades are attained by the addition of processing oils and additionally ethylene copolymers including specifically ethylene vinyl acetate (EVA), ethylene methyl acrylate (EMA), ethylene n-butyl acrylate (EnBA), and ethylene ethyl acrylate (EEA) copolymers may be blended with the polymer to achieve (unspecified) suitable results. U.S. Pat. Nos. 6,011,095 and 6,414,056 exemplify the specific use of polyphosphoric acid (PPA) and/or superphosphoric acid (SPA) as the acid adjuvant in the promotion of the chemical bonding between asphalt and the available epoxy groups of ethylene/methyl acrylate/glycidyl methacrylate terpolymer (EMAGMA) and ethylene/n-butyl acrylate/glycidyl methacrylate terpolymer (EnBAGMA), respectively.